Mining technologies remove valuable geological materials from the earth. The materials of value removed from the earth by mining can include rocks, stones, minerals, mineraloids, and the like. The term “rock” or “stone” refers to a naturally occurring solid aggregate comprising minerals or mineraloids. A mineral is a naturally occurring solid chemical substance formed through biogeological processes and having a specific set of chemical and physical characteristics, including a particular chemical composition, a crystalline or highly ordered physical structure, and discernible properties derived therefrom. Mineraloids have more variability in their chemical compositions, and they do not demonstrate crystallinity. Minerals, mineraloids or rocks having economic value can be extracted from the earth using a variety of mining technologies. Target materials of value recovered by mining include minerals, mineraloids and rocks such as precious and base metals, iron, uranium, limestone, rock salt, potash, oil shale, oil sands, coal, diamonds, an the like.
Mining technologies typically remove large amounts of non-valuable geological materials from the earth along with the valuable material(s) of interest. The target material is then separated from the non-valuable material. The target material can undergo further processing to concentrate it or to make it otherwise more suitable for its end-use. The non-valuable material (“gangue”) remaining after the extraction and processing of the target material becomes waste, requiring disposal. This mining waste comprises geological substances such as topsoil, overburden and waste rock that are separated during the earlier stages of processing, as well as tailings remaining after the target material has been largely recovered.
Processing technologies to extract target materials from the gangue can begin by crushing the raw mined substances into smaller fragments. The crushed fragments are then separated by size, and are further treated so that the target materials can be separated physically or chemically from the gangue. Water-driven separation processes are often used to facilitate removing the gangue from the target material. Crushing and separation techniques produce fragments of varying sizes, from large lumps to fine particles. Larger masses of waste materials can be removed mechanically during processing. The fine particles, comprising both target materials and waste, can become suspended in the aqueous fluid stream used for separation processing. This fluid stream, carrying the suspended fine particles (“fines”) ultimately exits the processing plant as a liquid waste slurry requiring disposal. This waste slurry contains predominately low-value substances to be discarded as wasted, but it also contains fine particles of target materials that have not been captured by the various extraction processes. In certain cases, the waste slurry is stored in artificial reservoirs, such as tailings ponds. In other cases, the waste slurry is allowed to dessicate, so that the waste products, containing valuable target materials, are stored in dry piles.
Tailings from mining processes thus can contain varying amoungs of valuable target materials, depending, inter alia, upon the initial content of the target material in the mined substance, and upon the efficiency of the separation and extraction processes. Economics can dictate the extent to which the target material is removed during such processing. A precious metal like gold may be removed to a large degree from its ore, with only a small percent entering the tailings. The amount of precious metal in tailings, although small, can nonetheless have great economic value. By contrast, less valuable target materials may be of higher concentrations in tailings, because it does not make commercial sense to extract them more thoroughly. Though such target materials (e.g., coal) may have less intrinsic economic value than other target materials (e.g., precious metals), the relatively high concentration of the lower-value target materials in the tailings waste can make their recovery commercially viable. Processes for recovering target materials from tailings waste therefore offer the potential for extracting additional economic value from what has traditionally been discarded. Moreover, techniques for recovering target materials from tailings waste can offer alternatives to the traditional methods for tailings management, including storage in ponds or behind dikes or embankments or in spent mines (all with well-known environmental implications).
As an example, the mining and processing of coal results in tailings containing significant amounts of this valuable target material. In the U.S., around 1.1 billion tons of coal are mined annually. Approximately 8-10% of mined coal ends up as fines, which are not captured by the production process and typically end up in slurry impoundments. The U.S. Department of Energy estimates that 2 billion tons of coal fines currently exist in impoundment areas. Recovering coal as the target material from such tailings offers a significant economic and environmental opportunity. Recovering coal fines from slurry streams and impoundments can improve coal mining efficiency, because the recovered coal fines can be processed and used for fuel. In addition, recovering coal fines can reduce the waste footprint of coal mining operations. However, recovering coal fines from waste streams in an easy and efficient manner, and in a condition where the recovered coal is ready for transport and other uses, remains a great challenge.
The mining and processing of coal illustrates how such tailings are formed, and how they come to contain sufficient target material to be economically valuable. Coal as it is recovered from the mine is initially crushed to reduce its size and to free it up from the larger mineral inclusions, using a variety of size reduction techniques like crushers, rotary breakers or other similar devices. After crushing and size separation, the smaller coal fragments can be exposed to an aqueous stream such as froth flotation that separates the coal from the minerals that surround it. Using froth flotation, for example, crushed coal fragments can be mixed with water and other additives, then exposed to streams of air bubbles. Coal that is carried to the surface in the froth can be skimmed off, screened and dewatered for commercial uses, while the minerals sink to the bottom. After these water-driven separation processes, fine particles (“fines”) comprising inorganic and organic materials (coal, clay, rocks, minerals, and the like) remain suspended in the aqueous slurry. The fines comprising coal particles can be termed “coal fines.” Separating fines from the aqueous suspending medium is difficult, as the fines tend to remain suspended unless energy-intensive processes are employed to recover them. In coal mining and processing, significant quantities of coal fines are created that require disposal and handling. Similar aqueous streams containing suspended fines are produced during the mining and processing of other valuable mined materials.
As mentioned above, the amount of suspended fines in the aqueous slurry stream can vary, depending upon the type of valuable material being mined and depending upon how it is processed. As an example, the amount of coal fines in the slurry stream will vary by site depending on the efficiency of the coal processing facility. Other fines in the coal-fines slurry include clays and fine mineral materials. Treating the slurry to remove all suspended solids and recover clarified water is a difficult problem. Additionally, it is highly desirable to recover the high value mined material, such as coal fines in this example, from the aqueous slurry. Older, less efficient coal processing facilities tend to have larger quantities of valuable coal fines ending up in the slurry stream. To increase the overall efficiency of coal processing, the ability to selectively recover the valuable coal fines from the slurry is needed. Recovering the coal-rich fines from the slurry by mechanical means is difficult, expensive, and inefficient.
There remains a need in the art, therefore for systems and methods for recovering valuable materials from mining tailings. Coal offers an example of such a valuable mined material. As would be apparent to skilled artisans, though, the problem of recovering valuable mined materials from tailings waste extends throughout the mining industry. There exists a need in the art for systems and methods capable of recovering a variety of valuable mined materials, including but not limited to precious metals, base metals, rare earth minerals, and the like, from the tailings waste in which they are contained.